A typical structure of a conventional polymer electrolyte fuel cell will be described:
A fuel cell using a polymer electrolyte generates electric power and heat simultaneously by electrochemical reaction of a fuel gas containing hydrogen with an oxidant gas containing oxygen such as air.
FIG. 1 shows a schematic sectional view of an electrolyte membrane-electrode assembly (MEA) 15 of the polymer electrolyte fuel cell. The MEA comprises a hydrogen ion-conductive polymer electrolyte membrane 11 and a pair of electrodes 14 arranged on both sides of the electrolyte membrane 11. Each of the electrodes comprises a catalyst layer 12 in contact with the electrolyte membrane 11 and a gas diffusion layer 13 in contact with the catalyst layer 12. The catalyst layer 12 is formed from a mixture of the hydrogen ion-conductive polymer electrolyte and carbon particles carrying platinum-group metal catalyst particles thereon.
The electrolyte membrane 11 used in the present invention is a copolymer of perfluorocarbon sulfonic acid and polytetrafluoroethylene (hereinafter referred to as perfluorocarbon sulfonic acid), for example, a Nafion film produced by Du Pont in the US, and the like having the following formula:
wherein 5≦x≦13.5, y=1000, m=1 and n=2.
The gas diffusion layer 13 used is a conductive porous substrate such as carbon paper having gas permeability. The conductive porous substrate may be made water-repellent.
A sealing member such as a gasket is arranged around the electrode 14 with the electrolyte membrane 11. The sealing member is intended to prevent the fuel gas and the oxidant gas for supply to the electrodes from being leaked to the outside or mixed with each other. The sealing member is ted with the MEA.
FIG. 2 shows a schematic sectional view of a unit cell 23 of the polymer electrolyte fuel cell. The unit cell 23 comprises the MEA 15 and a pair of conductive separators 21 arranged on both sides of the MEA. The conductive separator 21 serves to mechanically fix the MEA. On the face of the separator 21 in contact with the MEA 15 formed is a gas flow channel 22 for supplying the fuel gas or the oxidant gas to the electrode and carrying away redundant gas and water produced through an electrode reaction. Although the gas flow channel 22 can be provided independently of the separator 21, a typical process is to arrange a groove in the surface of the separator so as to form the gas flow channel 22. By supplying the fuel gas to one gas flow channel and supplying the oxidant gas to the other gas flow channel, electromotive force of about 0.8 V can be generated out of one unit cell 23.
Normally, plural unit cells 23 are connected in series to obtain a voltage of several volts to several hundreds volts. Therefore, the gas flow channels 22 are formed on both faces of the separator 21, and then the unit cells are connected in series in order: separator/MEA/separator/MEA.
The gas is supplied to the gas flow channel through a manifold. There are two types of manifolds: a manifold connecting several branches of a gas supply pipe directly to the gas flow channel (called an external manifold) and manifold in the form of through holes, arranged through the separator with the gas flow channel and communicating with the inlet and outlet of the gas flow channel (called an internal manifold).
Next, three functions of the gas diffusion layer will be described:
First, the gas diffusion layer has the function of diffusing the gas in order to supply the fuel gas or the oxidant gas uniformly to the catalyst particles in the catalyst layer.
Secondly, the gas diffusion layer has the function of promptly carrying away water produced in the catalyst layer into the gas flow channel.
Thirdly, the gas diffusion layer has the function of conducting electrons involved in the reaction.
The gas diffusion layer is required to have excellent gas permeability, steam permeability and conductivity Conventionally, therefore, the gas diffusion layer has been made from a conductive porous substrate with pores developed therein such as carbon paper, carbon cloth or carbon felt in order to secure the gas permeability. The steam permeability is secured by dispersing water-repellent polymers in the gas diffusion layer. Further, the conductivity is secured by using a conductive material such as carbon fiber, metal fiber or a carbon fine powder for the gas diffusion layer.
Next, four functions of the catalyst layer will be described:
First, the catalyst layer has the function of supplying the fuel gas or the oxidant gas supplied from the gas diffusion layer to a reaction site.
Secondly, the catalyst layer has the function of promptly conducting hydrogen ions involved in the reaction to the electrolyte membrane.
Thirdly, the catalyst layer has the function of conducting electrons involved in the reaction.
Fourthly, the catalyst layer has the function of promptly advancing a redox reaction by providing a large reaction area and a highly-active catalyst.
The catalyst layer is required to have excellent gas permeability, hydrogen ion permeability and conductivity, but also to provide an excellent reaction site for the reaction. Conventionally, therefore, the gas permeability is secured by forming a catalyst layer precursor from a mixture of a pore-producing agent and carbon particles with pores developed therein, and removing the pore-producing agent. The hydrogen ion permeability is secured by dispersing the polymer electrolyte in the vicinity of the catalyst particles to form a hydrogen ion-conductive network. Further, the conductivity is secured by forming a catalyst carrier from a conductive material such as carbon particles or carbon fiber. Moreover, by making several-nm-size fine catalyst particles comprising a platinum-group metal on a carrier, the dispersibility of the catalyst particles in the catalyst layer are enhanced, enabling provision of a favorable reaction site.
The following problems exist concerning the electrode of the conventional fuel cell:
First, there is a problem that the gas permeability, steam permeability and conductivity of the electrode are properties mutually contradictory. For example, when the gas diffusion layer is made of carbon fiber with a small diameter or the porosity of the gas diffusion layer is increased in order to enhance the gas permeability of the electrode, the conductivity of the gas diffusion layer decreases. When the water-repellent polymers are added to the gas diffusion layer in order to enhance the steam permeability, the gas permeability and conductivity of the gas diffusion layer decrease. Thus, there is a need to make the mutually contradictory properties compatible by forming the gas diffusion layer, not from a single material, but from a combination of a layer comprising the carbon fiber and a layer comprising the carbon particles and the water-repellent polymers. Also it has been proposed to use of a surfactant to obtain a favorable dispersed state of the carbon particles and the water-repellent polymers. For example, the conventional process for producing the gas diffusion layer comprises the steps of preparing a water-repellent ink which includes carbon particles, water-repellent polymers, a surfactant and a dispersion medium and applying the water-repellent ink to the conductive porous substrate has been studied. There are, however, few examples of detailed studies regarding the effect of the surfactant.
Japanese Laid-Open Patent Publication No. Hei 11-335886, Japanese Laid-Open Patent Publication No. Hei 11-269689, Japanese Laid-Open Patent Publication No. Hei 11-50290, Japanese Laid-Open Patent Publication No. Hei 10-092439 and Japanese Laid-Open Patent Publication No. Hei 6-116774 disclose octyl phenol ethoxylate belonging to alkylphenol group as the surfactant for dispersing the water-repellent polymers in the water-repellent ink. Further, Japanese Laid-Open Patent Publication No. Hei 6-036771 discloses: anionic surfactants such as fatty acid soap, alkylbenzene sulfonate, alkylaryl sulfonate and alkylnaphthalene sulfonate; cationic surfactants such as alkylamine salt, amide-bonded amide salt, ester-bonded amine salt, alkylammonium salt, amide-bonded ammonium salt, ester-bonded ammonium salt, ether-bonded ammonium salt, alkylpyridinium salt and ester-bonded pyridinium salt; amphoteric surfactants such as a long-chain alkylamino acid; and nonionic surfactants such as alkyl aryl ether, alkyl ether, alkylamine fatty acid glyceric ester, anhydro sorbitol fatty acid ester, polyethylene imine and fatty acid alkylolamide. In the examples of the above documents, however, only octyl phenol ethoxylate belonging to alkylphenol was studied. Therefore, the above documents simply introduce a variety of common surfactants extensively, and effects of these surfactants when used for an electrode of a fuel cell are unclear.
Surfactants of alkylphenol group are environmental hormones suspected of having the endocrine-disrupting function. For this reason, such surfactants raise a safety issue, safety of the production of the electrode and the MEA, safety of a final product in a case where a trace quantity of alkylphenol is left, and safety in waste disposal of the final product. In order to reduce these safety hazzard, extraction of the surfactant by solvent is necessary. However, this extraction requires liquid waste disposal, a scrubber and the like, thereby raising a problem of increased cost.
Meanwhile, when no surfactant is used, the following problem may arise. First, the water-repellent polymers do not sufficiently disperse and are unevenly distributed in the electrode, making it impossible to control the water content of the electrode and to secure sufficient electrode strength. Second, since stability of the water-repellent ink decreases, the solid matter concentration thereof does not become uniform and a pipe or a pump of an applying apparatus is clogged with the water-repellent ink in the production process of the gas diffusion layer. As a result, variation or defect of a coating of the water-repellent ink occurs, causing the discharge performance of the electrode to deteriorate.
Next, there is a problem with the electrode of the conventional fuel cell that the water content increases with the passage of time. This is because water is produced through the electrode reaction, and further, the reaction gas contains water for humidification. When the water content of the catalyst layer and the gas diffusion layer increase, micropores as gas channels become clogged, which causes insufficient supply of the gases to the electrodes, leading to deterioration in cell performance. On the other hand, when the humidity of the gas is decreased or humidification is suspended for a long time, the water content of the polymer electrolyte and the electrolyte membrane in the MEA decrease to cause the hydrogen ion-conductive network to deteriorate, leading to deterioration in cell performance. The cause of such a phenomenon lies in the difficulty of controlling the water content of the electrode due to insufficient moisture retention of the conventionally-used electrode.
For this reason, Japanese Laid-Open Patent Publication No. Hei 10-334922 proposes making a moisture retentive agent comprising sulfuric acid or phosphoric acid contained in the catalyst layer. Since sulfuric acid and phosphoric acid are apt to vaporize, control of the water content is difficult. Further, the use of sulfuric acid or phosphoric acid may raise a further problem, corrosion of the structural members of the fuel cell system. Moreover, Japanese Laid-Open Patent Publication No. 2000-251910, Japanese Laid-Open Patent Publication No. 2001-15137, Japanese Examined Patent Publication No. Hei 10-52242 and Japanese Laid-Open Patent Publication No. 2000-340247 disclose a means of controlling the water content of the catalyst layer and the gas diffusion layer from the outside of the electrode or the catalyst layer by the use of a polymer water-absorptive sheet. However, it is not possible to control local water content of the catalyst layer and the gas diffusion layer in the method for controlling the water content of the catalyst layer and the gas diffusion layer from the outside.
Next, there is another problem with the electrode of the conventional fuel cell and that is it is difficult to produce a catalyst layer with catalyst particles evenly distributed therein. The catalyst layer is required to simultaneously have high gas diffusibility, conductivity, catalyst activity and hydrogen ion permeability. To satisfy this requirement, it is necessary to evenly distribute the catalyst particles along the plane of the catalyst layer.
A typical catalyst layer is formed by applying the catalyst ink. Examples of the applying method may include a screen-printing method, a spraying method, a gravure-printing method and a coater method. The catalyst ink is prepared by mixing carbon particles carrying catalyst particles thereon, a hydrogen ion-conductive polymer electrolyte and a dispersion medium such as water or alcohol. It is common that the catalyst ink is further mixed with a thickener for facilitating the application.
Generally, the viscosity of the catalyst ink is measured by a single shear rate, and in the field of the catalyst ink of the fuel cell. There have been few detailed studies on thixotropy conducted by changing the shear rate.
In Japanese Laid-Open Patent Publication No. Hei 8-235122, thickeners with a high viscosity such as fluorine-atom-containing alcohols are used to control the catalyst ink's viscosity. Further, there also is a method in which a thickener with a high viscosity such as glycerol is used. Since the thickener needs to be removed from the catalyst layer, the use of the thickener necessitates heating of the catalyst layer at high temperature after the formation thereof Problems may hence arise in that the hydrogen ion-conductive polymer electrolyte in the catalyst ink deteriorates and that the production cost of the electrode increases due to a high heating temperature of 100° C. or higher.
As indicated in Japanese Laid-Open Patent Publication No. Hei 11-16586, there is another method in which catalyst ink is prepared with no thickener added thereto and the viscosity of the obtained catalyst ink is adjusted by heating. In this method, however, it is difficult to control the amount of evaporating dispersion medium, and thus it is not easy to adjust the viscosity. Moreover, a problem may arise that the hydrogen ion-conductive polymer electrolyte in the catalyst ink denatures or agglomerates in heating.